1. Field of the Invention
The present invention relates to an inkjet printing apparatus which uses a plurality of print heads capable of ejecting ink through ink ejection ports to print an image on a print medium, the apparatus comprising a recovery unit capable of sucking and discharging the ink from each of the ink ejection ports in the plurality of print heads.
2. Description of the Related Art
As printing apparatuses, ink jet printing apparatuses are commonly used which print an image by ejecting ink to a print medium through a plurality of ink ejection ports formed in a print head. Methods for ejecting ink use electrothermal conversion element (heater), piezo element, or the like. If electrothermal conversion element is used, the electrothermal conversion element generates thermal energy in response to driving pulse so that the resulting bubbling energy can be utilized to eject ink droplet through the ink ejection port.
Some of the ink jet printing apparatuses uses, as a print head, a multi-nozzle head including integrated multiple nozzles each composed of an ink ejection port, an ink channel, and the like in order to improve an image printing speed. Furthermore, in a certain type of ink jet printing apparatuses (line printers), the print head is formed into a line head extending in a direction crossing a print medium conveying direction, and a plurality of the print heads are arranged along the print medium conveying direction. Ink is then ejected through ejection ports in the line heads in conjunction of conveyance of a print medium.
Printing apparatuses configured to print images on print media need to print images of a high resolution at a high speed. The use of the above-described ink jet printing apparatuses, including the line printers, allows this need to be satisfied.
On the other hand, since the ink jet printing apparatus handles ink, which is a fluid, the physical properties of the ink in the print head may vary. The variation in physical properties includes a variation in the viscosity of the ink associated with an environmental temperature. Furthermore, depending on the time for which the printing apparatus is left inactive, moisture in the ink may evaporate to increase the viscosity of the ink. Such a variation in ink viscosity seriously affects a recovery process described below and eventually a quality of the printed image.
As a mechanism for properly maintaining the ink ejection state of the print head, a suction recovery mechanism is known which sucks and discharges ink through the ink ejection ports of the print head (a suction recovery process). The suction recovery mechanism includes a cap configured to cap the ink ejection ports of the print head and a suction pump (negative-pressure supply source) configured to generate a negative pressure to be introduced into the cap in the capping state via a tube (suction path). The suction recovery mechanism thus sucks and discharges the ink through the ink ejection ports.
Japanese Patent Laid-Open No. H11-78065 (1999) describes a suction recovery mechanism that can perform a suction recovery process according to differences in the channel resistance of an ink channel among the print heads resulting from manufacturing errors. That is, the ink suction and discharge amount of each of the print heads is controlled according to the differences in the channel resistance of the ink channel among the print heads. The control also deals with a variation in ink viscosity caused by a variation in the physical properties of the ink depending on the environmental temperature.
Furthermore, Japanese Patent Laid-Open No. 2007-22036 describes a configuration that varies the channel resistance of a supply path through which ink is refilled, in order to adjust a difference in the amount of ink sucked and discharged which difference is caused by a difference in the opening area of the ink ejection port of the print head.
If a plurality of caps arranged opposite the respective plural print heads are each connected to one suction pump via a suction path, placing the pump and each of the caps at an equal distance from each other is difficult owing to, for example, restrictions on the printing apparatus required for miniaturization. Thus, the lengths of the suction paths vary. In this case, the ink suction and discharge amount of the print head corresponding to each cap may vary depending on the positional relationship between the cap and the suction pump. This is because the length or bending degree of the tube (suction path) connecting the cap and the suction pump together may vary depending on the positional relationship between the cap and the suction pump, causing the channel resistance to vary among the caps. For example, a cap located away from the suction pump is connected to the suction pump via a relatively long tube, which offers a relatively high channel resistance. On the other hand, a cap located close to the suction pump is connected to the suction pump via a relatively short tube, which offers a relatively low channel resistance.
Even if each tube has the same length, a tube having a large number of bent portions offers a high channel resistance. Because the bent portion has a low fluidity of a bubble kept in the tube as a foreign matter, and the bubble performs as a buffer to increase the channel resistance. Therefore, as long as there are a plurality of tubes, it is difficult to uniform the channel resistance of each of the tubes because of the difference of position on which each tube is disposed or the difference of form of each tube.
The ink suction and discharge amount of the print head corresponding to each cap varies as a result of such a difference in the channel resistance of the tube. If a tube has a high channel resistance, the print head corresponding to the tube may fail to achieve a sufficient suction and discharge process. Assuming that the negative pressure to be introduced into each of the tubes is set based on the high channel resistance, a higher negative pressure than required is applied to the print head corresponding to a tube having a low channel resistance, and from the print head an increased amount of ink may be sucked to decrease the usability of the ink. Especially, in a case where an elongated print head is used, such a decrease in the usability of the ink became conspicuous, the ink suction and discharge amount of the elongated print head increase further.